Separation Vessels For Use In Polymerization Processes And Methods For Cleaning Same

ABSTRACT

Both a system and method for cleaning a low pressure separation vessel of a high pressure polyethylene polymerization plant are provided. The system includes a polytetrafluoroethylene lining that covers the interior surfaces of the vessel, and a cover mounting assembly including an annular clamp for detachably mounting a cover over the vessel. The mounting assembly includes a clamp actuator for quickly securing and releasing the cover with respect to a top rim of the vessel. The vessel is drained of liquid polyethylene and allowed to cool to ambient temperature, thus creating a frozen “skin” of polyethylene around the interior surfaces of the vessel. The clamp actuator releases the cover. The polyethylene skin is peeled off the interior sides the vessel and gathered up at the top to form a neck, thus peeling the polyethylene skin away from the polytetrafluoroethylene lining along with any degraded polymers or other impurities that have accumulated on the interior surfaces of the vessel.

FIELD OF THE INVENTION

This invention generally relates to a high pressure polymerizationprocess for the manufacture of polyethylene, and is specificallyconcerned with a system and method for cleaning a low pressureseparation vessel of a polyethylene polymerization plant that providesrapid and effective in situ removal of impurities that accumulate withinthe vessel.

BACKGROUND OF THE INVENTION

In the manufacture of ethylene polymers, ethylene gas is compressed intoa supercritical fluid and then heated. The hot supercritical ethylene isthen admitted into a tubular polymerization reactor, along with a supplyof a chemical initiator and a modifier. The chemical initiator initiatespolymerization of the free radical ethylene, while the modifier controlsthe molecular weight of the resulting polyethylene. Since only about 40%of the ethylene monomers react, the resulting polyethylene product thatis discharged from the reactor is a mixture of ethylene polymersintermixed with unreacted ethylene. Consequently, it is necessary toseparate the polymers from the ethylene. To this end, a high pressureseparator vessel and a low pressure separator vessel are seriallyconnected to the outlet of the polymerization reactor. The high pressureseparator vessel initially receives the reactor product from the reactorat about 40,000 psi. The reactor contents are depressurized to about4000 psi through a control valve into the high pressure separatorvessel, which separates most of the polymer from the ethylene. Theresulting polyethylene product still contains about 10% unreactedethylene, and is admitted to the low pressure separator vessel. Thelower pressure in this vessel results in the flashing away of theremainder of the unreacted ethylene from the product. The resultingpolyethylene is then admitted into an extruder for final processing.

During processing, the outer walls of the low pressure separator vesselare continuously heated by means of a steam jacket in order to maintainthe polyethylene product in a flowable liquid state. The applicants haveobserved that the non-Newtonian characteristics of the liquidpolyethylene flowing through the low pressure separator vessel resultsin a very slow flow rate at the interface between the liquidpolyethylene and inner surface of the vessel. The inner surface of thevessel is also where the interior temperature of the vessel is highestdue to its closeness to the steam jacket that surrounds the exterior ofthe vessel. The combination of the high temperature of the vessel innersurface and the long residence time of the liquid polyethylene over itresults in the production of degraded polymers on the inner surface dueto thermally-induced, cross-linking reactions. If these degradedpolymers are not periodically removed from the inner surfaces of the lowpressure separator vessel, they can contaminate the final polyethyleneproduct and degrade its appearance and film properties. The problem isworse in situations where a high clarity and purity polyethylene productis essential for the rendering of a particular final product, such asblown film products, medical applications and sensitive electricalapplications.

To solve this problem, polyethylene manufacturers typically periodicallyclean the inner walls of the low pressure separator vessel byhydroblasting every several months. But because hydroblasting takesseveral days and must be done with the vessel in a horizontal position,most polyethylene manufactures replace the fouled low pressure separatorvessel with a pre-cleaned, substitute separator vessel in order toreduce system downtime. Unfortunately, such a vessel replacementprocedure still takes about a day to implement due to the time requiredto (1) mechanically disconnect all of the interfaces of the fouledvessel with the other components of the polymerization plant, (2)exchange the multi-ton fouled vessel with a multi-ton cleaned vessel and(3) to re-connect all of the interfaces between the clean vessel and thepolymerization plant. Moreover, as the vessel weighs one or more tons,the step of exchanging the fouled vessel with a cleaned vessel must bedone by way of a slow and delicate crane operation in order to avoidbreakage or damage to the valves, pipes and other interface fittingsthat must be disconnected and reconnected.

SUMMARY OF THE INVENTION

Clearly, there is a need for an improved cleaning technique for a lowpressure separator vessel that is faster and that reduces the amount ofdowntime of the polymerization plant. Ideally, such a technique would beeasier and less expensive to implement, and would reduce the amount ofdowntime for cleaning operations necessary to maintain a high qualitypolyethylene product.

The invention is a system and method for cleaning a separation vesselthat fulfills all of the aforementioned needs. To this end, the systemof the invention generally comprises a low-stick lining, preferably apolytetrafluoroethylene lining, that covers at least a portion of theinterior surfaces of the vessel, and a detachable cover mountingassembly including a clamp for detachably mounting a cover over thevessel in a pressure-tight relationship. “Low-stick” for purposes ofthis specification and appended claims means a lining producing areduced tendency for polymer product, such as polyethylene orpolypropylene, to adhere to a surface so lined as compared to a surfacewithout such low-stick lining. The system may include a layer of metalapplied over the interior surfaces of the vessel to provide adhesionbetween the polytetrafluoroethylene lining and the interior surfaces ofthe vessel. The vessel lining is preferably a layer (i.e., film) ofpolytetrafluoroethylene having a dry film thickness that is preferablybetween about 0.02 and 0.20 mm, and the metal layer is preferably anickel layer between about 0.050 and 0.150 mm thick. As used herein “dryfilm thickness” means the thickness of the film after it has thoroughlydried (e.g., after all the solvent has evaporated and the film hascured). The mounting assembly includes a clamp actuator that secures andreleases the clamp into and out of a clamping position. The cover andthe top rim of the vessel may each include annular flanges which theclamp may capture and pull together when actuated into the clampingposition. The clamp actuator may include one or more hydraulic cylindersthat can rapidly secure and release the clamp into and out of a clampingposition. The mounting assembly preferably further includes a gasketthat provides a pressure-tight seal between the cover and the vesselwhen the clamp is secured by the clamp actuator. The system may furtherinclude a hoist for removing an impurity-laden layer of polyethyleneskin off of the polytetrafluoroethylene lining that covers the interiorsurfaces of the vessel, and a scraping tool, such as a wooden spatula,for initiating the peeling of a polyethylene skin off of the interiorsides of the vessel.

In the method of the invention, the low pressure vessel is emptied ofliquid polyethylene and the pressurized ethylene gas is bled off andrecycled into the reactor until ambient pressure is achieved. The vesselis then cooled through, e.g., exposure to the environment or,preferably, by circulating cooling water through the heat exchangerpanels on the vessel exterior, to substantially ambient temperature,which freezes the liquid polyethylene clinging to the interior surfacesof the vessel into a skin of solid polyethylene. The clamp is thendetached from the cover, which allows the cover to be quickly removedfrom the top of the vessel. The polyethylene skin is next peeled off ofthe sides of the low pressure vessel and gathered up at the top to forma neck, which in turn is connected to the hoist of the system. The hoistlifts the knot upwardly, which peels the polyethylene skin away from thepolytetrafluoroethylene lining along with any impurities that haveaccumulated on the interior surfaces of the vessel. The hoist lifts theresultant bag-like polyethylene skin completely out of the vessel,thereby completing the cleaning of the vessel. The cover is thenre-positioned over the top end of the vessel, and the clamp isre-attached over the cover and the upper rim of the vessel to create apressure-tight seal between the cover and the vessel. The reactor isre-activated and the vessel is put back into production.

The cleaning process of the invention requires only about 2 hours toperform, in contrast to the full day required by the prior art method.Moreover, the invention obviates the need for two separate low pressureseparator vessels, and does not require disconnection and lifting andlowering steps that can damage the vessel. Finally, thepolytetrafluoroethylene lining that covers the interior surfaces of thevessel and, optionally, the cover, not only results in more thoroughcleaning when the polyethylene skin is lifted off of the interiorsurfaces, but also promotes a higher degree of flow on the innersurfaces of the vessel during the manufacture of the polyethylene, thusreducing the number of vessel cleanings required to maintain a highquality product.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a polyethylene plant having a lowpressure separator vessel that the system of the invention is appliedto;

FIG. 2 is an enlarged side view of the low pressure separator vesselshown in FIG. 1;

FIG. 3 is an enlarged view of the cross section of the vessel wall areacircled in phantom in FIG. 2, illustrating both thepolytetrafluoroethylene lining and nickel coating of the system of theinvention;

FIG. 4 is a plan view of the view of the low pressure separator vesselshown in FIG. 1 with the cover removed, illustrating the cover mountingassembly in a closed state in solid lines and in an open state inphantom lines;

FIG. 5 is an enlarged view of an end of the cover mounting assemblyalong the line 5-5 in FIG. 2, illustrating the relationship between theannular flange that circumscribes the upper rim of the vessel, the clampof the cover mounting assembly, and the clamp securing bolt;

FIG. 6A is a partial, side cross sectional view of the cover mountingassembly shown in an open state without the clamp;

FIG. 6B is a partial, side cross sectional view of the cover mountingassembly shown in a closed state without the clamp;

FIG. 6C illustrates the cover mounting assembly shown in FIG. 6B withthe clamp;

FIG. 7 illustrates the initial steps of the cleaning method of theinvention after the cover of the vessel has been removed to provideaccess to a polyethylene skin that has hardened over the inner surfaceof the vessel;

FIG. 8 is a top, cross sectional view of the vessel along the line 8-8,illustrating how the polyethylene skin dimples away from the innersurface of the low pressure separator vessel as a result of the coverbeing removed;

FIG. 9 illustrates the cleaning method steps of separating thepolyethylene skin from the inner surfaces of the vessel with spatulasand gathering the top of the skin into a neck; and

FIG. 10 illustrates the final steps of cleaning method of the inventionof lifting polyethylene skin from the inner surfaces of the vessel witha hoist.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic of a polymerization plant 1 of the type thatincludes the low pressure separator vessel 15 that the cleaning systemand method are applied to. The low pressure separator vessel typicallyoperates at a pressure in the range of from 0.1 to 20 barg, morepreferably from 0.1 to 5 barg, yet more preferably from 0.1 to 2 bargand especially preferably from 0.1 to 0.9 barg (barg=bar gauge, that is,pressure in excess of atmospheric). The plant 1 includes an ethylenefeed line 2 which supplies fresh ethylene to a primary compressor 3. Theethylene discharged from the primary compressor 3 flows via conduit 4having a valve 4 a to a secondary compressor 5. Also entering thesecondary compressor 5 is a stream of fresh modifier(s) and/or optionalcomonomer(s) and a stream of recycled ethylene. The fresh modifierstream is supplied by a separate modifier pump 6. The recycled ethylenecomes from the high pressure recycle system 7.

The secondary compressor 5 discharges compressed ethylene in fivestreams 8 a, 8 b, 8 c, 8 d, and 8 e. Stream 8 a accounts for 20% of thetotal ethylene flow. Stream 8 a is heated by a steam jacket (not shown)which heats the ethylene, prior to entry into the front end of thetubular reactor 9. The four remaining ethylene side streams 8 b, 8 c, 8d, and 8 e each enter the reactor as sidestreams. Sidestreams 8 b, 8 c,8 d, and 8 e are cooled. The tubular reactor 9 is also shown with sixinitiator inlets 10 a to 10 f which are spaced at intervals alongreactor 9 and are fed from an initiator mixing and pumping station 11.

Downstream of the sixth initiator inlet 10 f and the sixth reactionzone, the tubular reactor terminates in a high-pressure, let-down valve12. The high-pressure, let-down valve 12 controls the pressure in thetubular reactor 9. Immediately downstream of the high-pressure, let-downvalve 12 is product cooler 13. Upon entry to the product cooler 13, thereaction mixture is in a phase-separated state. It exits into highpressure separator 14. The overhead gas from the high pressure separator14 flows into the high pressure recycle system 7 where the unreactedethylene is cooled and returned to the secondary compressor 5.

The polymer product flows from the bottom of the high pressure separator14 into the low pressure separator 15, separating almost all of theremaining ethylene from the polymer. That remaining ethylene istransferred either to a flare (not shown) or a purification unit (notshown) or is recycled via the primary compressor 3 from the productseparation unit to the secondary compressor. Molten polymer flows fromthe bottom of the low pressure separator 15 to an extruder (not shown)for extrusion, cooling and pelletizing.

With reference now to FIG. 2, the separator vessel 15 includes a vesselbody 17 having a product inlet 19 mounted on its side for receiving thepolymer product from the high pressure separator 14. Vessel body 17includes a cylindrical section 18 a that ends in a frustro-conicalsection 18 b at its bottom that functions to funnel purified liquidpolyethylene into an extruder (not shown). The cylindrical section 18 ais surrounded by a steam jacket (not shown) that continuously appliesheat to the vessel 15 during production to maintain the polyethyleneproduct in liquid form. The vessel 15 further includes a cover 21 thatis sealingly mountable over the top rim of the vessel body. Cover 21includes an overhead gas outlet 23 for conducting pressurized ethylenegas either back to the primary compressor 3 for recycling or to a flareor purification unit. Cover 21 further includes small and large rupturediscs 25 a, 25 b for relieving smaller or larger excess pressures inorder to avoid a catastrophic bursting of the vessel 15. Finally, thecover 21 includes a nitrogen purge line 27 for replacing air in thevessel with inert nitrogen prior to putting the vessel on-line, therebyavoiding any degradation of the polyethylene product as a result ofoxidation. The diameter of the vessel body 17 can range between 5 and 15feet (1.52 and 4.57 meters), while the length of the vessel body canrange between 10 and 40 feet, (3.05 and 12.2 meters).

With reference now to FIG. 3, the walls 30 of both the vessel body 17and the cover 21 may be formed from a curved plate 31 of either carbonsteel or stainless steel. The cleaning system of the invention includesa layer or lining 32 of a chemical having anti-stick characteristicswith respect to polyethylene over the inner surface of the walls 30.Preferably, layer 32 is comprised of polytetrafluoroethylene having adry film thickness between about 0.02 and 0.20 mm. More preferably, thedry film thickness of the polytetrafluoroethylene layer 32 of the vesselbody 17 is between about 0.02 and 0.07 mm, while the dry film thicknessof the polytetrafluoroethylene lining 32 of the cover 21 is betweenabout 0.04 and 0.15 mm. The preferred dry film thickness of thepolytetrafluoroethylene lining 32 of the cover 21 is greater due to thepresence of more tightly curved surfaces than the inner surface of thewalls 30 of the vessel body 17. When the steel plate 31 forming thewalls is formed from carbon steel, a layer 34 of a corrosion-resistantmetal 37, such as nickel is applied over the surface of the inner walls30 to provide a surface that the polytetrafluoroethylene layer 32 canadhere to. Without such a layer 34, the rust, corrosion and pitting thatinvariably forms on the surface of carbon steel over time would providesites where the polytetrafluoroethylene layer 32 would start peeling offof the inner surfaces of the walls 30. Such a layer 34 of nickel ispreferably applied by electrodeposition to a thickness between about0.050 and 0.150 mm. When the steel plate 31 forming the walls is formedfrom stainless steel, no layer 34 of a corrosion-resistant metal isnecessary, and the polytetrafluoroethylene layer 32 is applied directlyover the inner surface of such stainless steel plate with goodadherence. The optional inclusion of the layer 34 of acorrosion-resistant metal in the system of the invention advantageouslyallows the system to be retrofitted onto carbon steel, low pressureseparator vessels 15 used in older polyethylene plants.

With reference to FIG. 4, the cleaning system further includes a covermounting assembly 36 for detachably and sealingly mounting the cover 21to the upper rim 37 of the vessel body 17. To this end, the covermounting assembly 36 includes a clamp 38 formed from a pair of opposing,semicircular clamp members 39 a, b that are movable into and out of aclamping position by means of a pair of hydraulically-controlled clampactuators 40 a, b. The semicircular clamp members 39 a and 39 b aresupported by a pair of brackets 42 a, b and 42 c, d, respectively.

Each of the support brackets 42 a, b and 42 c, d includes a slot 44which slidably receives a guide pin 46 connected to one of the clampmembers 39 a, b. Each of the clamp actuators 40 a,b includes a pair ofhydraulic pistons 50 a, b and 50 c, d for moving the semicircular clampmembers 39 a and 39 b from a non-clamping position (illustrated inphantom) that allows the cover 21 to be lifted off the rim 37 to aclamping position (illustrated in solid lines) that sealingly mounts thecover 21 over the upper rim 37. The slidable engagement between theguide pins 46 and the slots 44 in the support brackets 42 a, b and 42 c,d confines the movement of the semicircular clamp members 39 a, bbetween the positions illustrated in FIG. 4 when the hydraulic pistons50 a, b and 50 c, d of the clamp actuators 40 a, b are operated.

As is illustrated in FIGS. 5 and 6A, the cover mounting assembly 36further includes annular flanges 52 and 56 circumscribing the bottom rimof the cover 21 and the top rim 37 of the vessel body 17, respectively.The top wall 54 of annular flange 52 and the bottom wall 58 of theannular flange 56 are slightly tapered in opposite directions as shown.A ring-shaped gasket 60 is provided between the bottom rim of the cover21 and the top rim 37 of the vessel body 17. The lower wall of theannular flange 52 terminates in a circular lip 62 that is complementaryin shape to an annular recess 66 present in the upper wall of theannular flange 56B. When the cover 21 is positioned over the rim 37 ofthe vessel body 17, the gasket 60 is seated between the circular lip 62of the annular flange 52 and the annular recess 66 of the annular flange56, as is shown in FIG. 6B.

FIG. 6C illustrates how the clamp 38 of the cover mounting assembly 36compresses the gasket 60 between the circular lip 62 and the annularrecess 66. Specifically, each of the semicircular clamp members 39 a, bincludes an annular recess 66 having opposing inner side walls 68 a, bwhich are slightly tapered at the same angles as the top wall 54 of theannular flange 52 and the bottom wall 58 of the annular flange 56.Consequently, when the hydraulic pistons 50 a-d of the cover mountingassembly 36 are actuated to retract the clamp 38 into the clampingposition illustrated in FIG. 4, the inner side walls 68 a, b wedginglyengage the top wall 54 and the bottom wall 58 to squeeze the upper andlower annular flanges 54 and 58 together, thereby compressing the gasket60 into sealing engagement between the circular lip 62 and annularrecess 66.

Finally, in order to lock the semicircular clamp members 39 a, b intothe clamping position shown in FIG. 4, the cover mounting assembly 36includes a combination of mounting lugs 70 a-d and locking bolts 72, asbest seen in FIG. 5. Each of the locking bolts 72 includes a ring-shapedend 74 pivotally mounted in each of the lugs 70 a and 70 c, and athreaded end 75 receivable into a recess in each of the lugs 70 b and 70d. When the hydraulic pistons 50 a-d are actuated, both of the lockingbolts 72 may be pivoted into the position illustrated in FIG. 4. Alocking nut 76 may then be screwed over the threaded ends 75 in order tomaintain the semicircular clamp members 39 a, b in the clamping positionafter the hydraulic pistons are de-actuated. To restore the semicircularclamp members 39 a, b back into the unclamping position, the oppositeprocedure is followed with the locking bolts 72 and the hydraulicpistons 50 a-d are actuated to withdraw the semicircular clamp members39 a, b back into the position illustrated in phantom in FIG. 4.

In addition to the previously-described cleaning system, the inventionalso includes a method for cleaning the low pressure separator vessel 15of a polyethylene plant 1. In the first steps of the cleaning method,the plant 1 is shut down, and an isolation valve (not shown) is closedthat prevents a further flow of polyethylene product into the inlet 19of the vessel body. The polypropylene product within the vessel 15 isallowed to drain out of the frustro-conical section 18 b at its bottominto the extruder.

While the vast majority of the polyethylene product will exit the vesselduring the drainage step, some of the product will cling to the innerwalls 30 of both the vessel body 17 and the cover 21 due the previouslydescribed non-Newtonian flow characteristics of the liquidpolypropylene. After the drainage step is completed, another valve (alsonot shown) is opened to vent the overhead gas outlet 23 of the cover 21to atmospheric pressure. At the same time, cold water is circulatedthrough the steam jacket that surrounds the vessel body 17 to “freeze”the remaining polypropylene into a skin layer 80 that covers thepolytetrafluoroethylene layer 32 that lines the inner surfaces of thewalls 30.

After the vessel 15 has cooled to ambient temperature, the nuts 76 ofthe locking bolts 72 are removed and the bolts 72 are pivoted into theunlocking position illustrated in FIG. 4. The hydraulic pistons 50 a-dof the cover mounting assembly 15 are actuated to withdraw thesemicircular clamp members 39 a, b back into the unclamping positionillustrated in phantom in FIG. 4. The cover 21 is then removed, as isillustrated in FIG. 7, and the skin layer 80 lining the inner surface ofthe cover 21 is manually peeled away and removed.

As is best seen in FIG. 8, when the cover 21 is removed, the skin 80(which is between about 1.5 and 3.0 cm thick) is torn long the interfacebetween the cover 21 and the upper rim 37 of the vessel body 17. Thetearing forces create separations 81 between the inner surfaces of thevessel walls 30 and the skin 80. With reference to FIGS. 9 and 10, theseseparations advantageously create starting points where the skin 80 maybe peeled back from the inner surfaces of the vessel walls 30 andgathered at its upper end into a neck 82. Specifically, spatulas 83formed by a wedge-like, wooden head 84 a connected to a long handle 84 bare inserted into the areas of separation 81 in order to separate theskin 80 from the vessel walls 30 such that the skin 80 forms acontinuous mass (e.g., a bag-like structure) that terminates at itsupper end in the neck 82. After the skin separation step has beencompleted, a noose 85 is lowered by means of a hoist 87 and is securedaround the neck 82 of the bag-like structure of skin 80. The hoist isthen raised to completely remove the bag-like structure of skin 80,which of course includes all of the degraded polymers that haveaccumulated over the inner surfaces of the vessel body 17 over time. Thecover 21 is then re-attached over the top rim 37 of the vessel body 17by the actuation of the hydraulic pistons 50 a-d, the locking bolts 72are locked in place, the overhead gas outlet 23 is reconnected to therecycling line shown in FIG. 1. Nitrogen is next admitted through thenitrogen purge line 27 to displace atmospheric oxygen out of the vessel15. The plant 1 is re-started, and the product inlet 19 is re-opened.

While the system and method of this invention have each been describedwith respect to a preferred embodiment, numerous modifications,equivalences and variations of this invention will become evident topersons of skill in the art. All such modifications, equivalences andvariations are encompassed within the scope of this invention, which islimited only by the appended claims and their equivalences.

1-29. (canceled)
 30. A separation vessel for use in a polymerizationprocess comprising: a detachable cover; and a lining that covers atleast a portion of the interior surfaces of the vessel, wherein suchlining comprises a layer which is low-stick with respect to a polymerresulting from the polymerization process.
 31. The system defined inclaim 30, wherein the polymer comprises at least one of polyethylene andpolypropylene.
 32. The system defined in claim 30, wherein the vessel isa component of a high pressure polymerization plant.
 33. The systemdefined in claim 30, wherein the cover has a mounting assembly adaptedto maintain the cover in a pressure-tight relationship with the vessel.34. The system defined in claim 33, wherein the mounting assemblycomprises a clamp.
 35. The system defined in claim 30, wherein saidlow-stick layer comprises polytetrafluoroethylene.
 36. The systemdefined in claim 35, wherein said low-stick layer has a dry filmthickness between about 0.02 and 0.20 mm.
 37. The system defined inclaim 30, further comprising a layer of metal applied over the interiorsurfaces of the vessel to promote adhesion between the low-stick liningand the interior surfaces of the vessel.
 38. The system defined in claim37, wherein said layer of metal is a layer of nickel between about 0.050and 0.150 mm thick.
 39. The system defined in claim 30, wherein at leasta portion of the interior surface of the cover comprises the low-sticklining.
 40. The system defined in claim 39, wherein the dry filmthickness of the low-stick lining on the interior surface of the coveris between about 0.04 and 0.15 mm.
 41. The system defined in claim 33,wherein the mounting assembly comprises a clamp actuator that securesand releases the clamp into and out of a clamping position that pullstogether said cover and a top rim of said vessel.
 42. The system definedin claim 41, wherein the mounting assembly further includes a gasketinterposed between the cover and the top rim of the vessel.
 43. Thesystem defined in claim 41, wherein the mounting assembly furtherincludes opposing flanges disposed along an outer periphery of both thecover and a top rim of the vessel, wherein the clamp captures and pullstogether said opposing flanges when in said clamping position.
 44. Thesystem defined in claim 30, further comprising a hoist that pulls away apolymer skin present on the low-stick lining that covers the interiorsurfaces of the vessel.
 45. The system defined in claim 41, wherein themounting assembly comprises a mounting mechanism including an annularclamp for detachably mounting said cover over the top rim of the vessel.46. The system defined in claim 44, further comprising at least onescraping tool for separating the polyethylene skin present on thepolytetrafluoroethylene lining that covers the interior surfaces of thevessel.
 47. The system defined in claim 45, wherein the clamp comprisesan actuator having at least one hydraulic cylinder for securing andreleasing the clamp into and out of a clamping position that pullstogether said cover and the top rim of the vessel.
 48. A method forcleaning a separation vessel containing a polymer comprising: providinga separation vessel having a detachable cover and a low-stick liningwith respect to said polymer that covers at least a portion of theinterior surfaces of the vessel; removing said cover; emptying thevessel of liquid polymer; cooling the vessel to form a skin of saidpolymer on said low-stick lining; and removing the polymer skin presenton the lining.
 49. A method for removing polymer from a separationvessel in a polymerization system comprising: providing a separationvessel having a removable cover and a low-stick lining with respect tosaid polymer that covers at least a portion of the interior surfaces ofthe vessel; removing said cover; stopping the flow of polymer to theseparation vessel; emptying the vessel of substantially all of theliquid polymer; cooling the vessel to form a skin of said polymer onsaid low-stick lining; and unpeeling the polymer skin present on thelining.